package com.tys.algorithm.advanced.code.class04;

/**
 * 小和问题：数组中每个数左边比自己小的数累加，整体计算累加和
 * num < num右边的数
 * 转换为：数num右边有多少个数大于num，产生几个num
 * O(N^2) -> O(N*logN)
 * 求小和
 */
public class Code02_SmallSum {
    public static int smallSum(int[] arr) {
        if (arr == null || arr.length < 2) {
            return 0;
        }
        return process(arr, 0, arr.length - 1);
    }

    // arr[L..R]既要排好序，也要求小和返回
    // 所有merge时，产生的小和，累加
    // 左 排序   merge
    // 右 排序  merge
    // merge
    public static int process(int[] arr, int L, int R) {
        if (L == R) {
            return 0;
        }
        //L < R
        int mid = L + ((R - L) >> 1);
        return process(arr, L, mid) //左边产生的小和
                + process(arr, mid + 1, R)  //右边产生的小和
                + merge(arr, L, mid, R); //合并产生的小和
    }

    //合并：排序并找到小和
    public static int merge(int[] arr, int L, int M, int R) {
        int[] help = new int[R - L + 1];
        int i = 0;
        int p1 = L;
        int p2 = M + 1;
        int ans = 0; //比归并排序多的
        while (p1 <= M && p2 <= R) { //不能越界
            //左组小于右组，产生的小和累加，不产生小和累加0
            //拷贝左组产生小和，拷贝右组不产生小和
            ans += arr[p1] < arr[p2] ? (R - p2 + 1) * arr[p1] : 0;
            help[i++] = arr[p1] < arr[p2] ? arr[p1++] : arr[p2++];
        }
        while (p1 <= M) { //p1没越界
            help[i++] = arr[p1++];
        }
        while (p2 <= R) { //p2没越界
            help[i++] = arr[p2++];
        }
        //导入原数组
        for (i = 0; i < help.length; i++) {
            arr[L + i] = help[i];
        }
        //返回小和
        return ans;

    }

    // for test
    public static int comparator(int[] arr) {
        if (arr == null || arr.length < 2) {
            return 0;
        }
        int res = 0;
        for (int i = 1; i < arr.length; i++) {
            for (int j = 0; j < i; j++) {
                res += arr[j] < arr[i] ? arr[j] : 0;
            }
        }
        return res;
    }

    // for test
    public static int[] generateRandomArray(int maxSize, int maxValue) {
        int[] arr = new int[(int) ((maxSize + 1) * Math.random())];
        for (int i = 0; i < arr.length; i++) {
            arr[i] = (int) ((maxValue + 1) * Math.random()) - (int) (maxValue * Math.random());
        }
        return arr;
    }

    // for test
    public static int[] copyArray(int[] arr) {
        if (arr == null) {
            return null;
        }
        int[] res = new int[arr.length];
        for (int i = 0; i < arr.length; i++) {
            res[i] = arr[i];
        }
        return res;
    }

    // for test
    public static boolean isEqual(int[] arr1, int[] arr2) {
        if ((arr1 == null && arr2 != null) || (arr1 != null && arr2 == null)) {
            return false;
        }
        if (arr1 == null && arr2 == null) {
            return true;
        }
        if (arr1.length != arr2.length) {
            return false;
        }
        for (int i = 0; i < arr1.length; i++) {
            if (arr1[i] != arr2[i]) {
                return false;
            }
        }
        return true;
    }

    // for test
    public static void printArray(int[] arr) {
        if (arr == null) {
            return;
        }
        for (int i = 0; i < arr.length; i++) {
            System.out.print(arr[i] + " ");
        }
        System.out.println();
    }

    // for test
    public static void main(String[] args) {
        int testTime = 500000;
        int maxSize = 100;
        int maxValue = 100;
        System.out.println("测试开始");
        boolean succeed = true;
        for (int i = 0; i < testTime; i++) {
            int[] arr1 = generateRandomArray(maxSize, maxValue);
            int[] arr2 = copyArray(arr1);
            if (smallSum(arr1) != comparator(arr2)) {
                succeed = false;
                printArray(arr1);
                printArray(arr2);
                break;
            }
        }
        System.out.println(succeed ? "Nice!" : "Fucking fucked!");
        System.out.println("测试结束");
    }

}
